Optical emission, far infrared, Raman, resonance Raman, and Brillouin spectroscopy are being used to measure vibrational frequencies in a variety of systems as a function of variables that lead to phase transitions or conformational changes. The interatomic and intermolecular potentials, and changes in them, can be determined from these measurements. The experiments thus provide information about the microscopic mechanisms behind phase transitions or conformational changes in molecular systems.